lib: fix SA warning in typesafe code

There's a nagging SA warning, at least with the scan-build
version we use in the FRR CI.

Signed-off-by: Mark Stapp <mstapp@nvidia.com>

lib/typesafe.h

@@ -309,6 +309,7 @@ static inline void typesafe_dlist_add(struct dlist_head *head,
 		struct dlist_item *prev, struct dlist_item *item)
 {
 	item->next = prev->next;
+	if (item->next)
 		item->next->prev = item;
 	item->prev = prev;
 	prev->next = item;